Organic transistors can be formed by using a flexible organic material capable of being applied as a coating and are expected to be used for driving elements of displays and IC tags. An organic transistor having a MOS-FET (metal oxide semiconductor field-effect transistor) structure includes a gate electrode, a gate insulating layer, a source electrode, a drain electrode, and an organic semiconductor layer which are formed over a substrate. A voltage is applied between the source electrode and the drain electrode from the gate electrode through the gate insulating layer to control an electric current passing through the organic semiconductor layer.
Organic TFTs (thin film transistors) have been studied actively in recent years, and it is expected that the organic TFTs will be applied, for example, to flexible displays by virtue of the flexibility of an organic semiconductor itself and the use of a resin substrate. In the organic TFTs, semiconductor layers are typically formed by vapor deposition. Pentacene, one of materials studied most actively, exhibits a mobility of 1 cm2/Vs or higher which is equal to or higher than that of amorphous silicon, and is expected to be used more widely in an organic semiconductor element.
To make the best possible use of the organic TFTs, various attempts have been made to form organic TFTs by coating application including the use of printing techniques with the aim of a low-cost process. Specifically, attempts have been made to form a film of polyalkylthiophene which is a polymeric semiconductor or a low-molecular-weight material such as a pentacene precursor by coating application. In addition to the semiconductor layer, the gate insulating layer has been studied to develop a material thereof soluble in a solvent such as a polymer which can be formed into a film by coating application.
Japanese Patent Laid-Open No. 2002-110999 has proposed a gate insulating film formed of a polymeric material having a high dielectric constant by including fine particles of metal oxide having a high dielectric constant dispersed in an amorphous insulator formed of a polymeric material having a high dielectric constant and containing a cyano group such as cyanoehylpullulan. The polymeric material having a high dielectric constant, however, generally has a low volume resistivity and is significantly polarized to have localized carriers, which tends to reduce the performance of the resulting transistor and have a poor surface property.
On the other hand, each of Japanese Patent Laid-Open Nos. 2005-72569 and 2005-26698 has proposed a high-performance organic transistor by using a gate insulating layer of stacked structure in which an insulating layer having a high dielectric constant is used for a first layer closer to a gate electrode and a flat polymeric material having a low dielectric constant is used for a second layer.